Server responses should be JSON

[Faterou]

Originally reported by: Sylvain Hallé (Bitbucket: sylvainhalle, GitHub: sylvainhalle)

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This is the case for all callbacks except the status page, which returns HTML. The status output should be changed into a JSON structure, which is then processed on the client side.

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• Bitbucket: https://bitbucket.org/sylvainhalle/cornipickle/issue/10